The present invention relates to an A.C. corona device for treating the surfaces of plastics films or metal foils.
In processing films as well as in manufacturing composite films, it is known in the prior art to subject the film surfaces to a corona discharge in order to render them suitable for printing or to increase their bond strengths. These methods involve passing the film to be treated over an electrically grounded support surface, such as a roll, a drum, or an endless belt, and subjecting the film surface which is not in contact with the support surface to an electrical corona discharge produced by supplying high-frequency, high-voltage A.C. to an electrode arranged at a distance from the support surface.
The known methods and devices operating according to this basic principle differ in reality only in the design of the support surface serving as a counter-electrode. These surfaces may, for example, comprise a central roll with several electrodes, several support rolls with corresponding electrodes or the like. The dielectric materials used to insulate the counter-electrode are, for example: mica, glass, ceramics, plastic films or special qualities of rubber. The electrodes conventionally employed may comprise a plate, wire, comb, knife, half-shell, spring or spindle-shaped electrode. The type of generator used may, for example, be a low-frequency, medium-frequency or high-frequency generator.
In addition, methods are known, such as those described in German Offenlegungsschrift No. 1,404,413, U.S. Pat. No. 2,864,755 and U.S. Pat. No. 2,802,085, in which the above-described conditions have been reversed, i.e., the film is subjected to an electrical corona discharge through an electrode insulated by means of a dielectric material.
The two basic systems just described (bare electrode/insulated counter-electrode or insulated electrode/bare counter-electrode) show general imperfections which are more or less troublesome in practice. When using a bare electrode and an insulated counter-electrode, one drawback, among others, is the relatively high cost resulting from damage to the insulation of the counter-electrode due to punctures, injuries such as knife cuts or the like or the introduction of moisture into the pretreating station. In such cases, the rolls which are usually insulated by special rubber layers, silicone layers, etc. must be sent to a rubberizing and vulcanizing plant for repair. As is known from experience, the repair procedure is technically quite involved and takes a prolonged period of time, so that the plant concerned is obliged to keep expensive spare parts in stock.
The problems outlined above have undoubtedly induced the development of the alternative method in which, preferably, cylinders or rolls covered with a dielectric material are used as electrodes, as already mentioned. The use of small electrode rolls certainly has some advantages. Apart from a simpler design and easier handling when mounting and dismounting, there is also reduction in the cost of repair achieved by using exchangeable dielectric linings in the form of hose materials which may be fitted or shrunk-fit in situ, instead of the vulcanizable, permanent insulating layers. As far as the effectiveness of the pre-treatment is concerned, i.e., the modification of surface (measured as the surface tension in mN/m) obtained on the treated substrate, the first-mentioned method is definitely superior to the alternative method, due to the possibility of combining the counter-electrode, necessarily designed as a continuous surface (insulated cylinder), with a point electrode of any shape (wire, comb, threaded pin or knife-shaped electrode, ect.). This is all the more understandable when one considers the preferred electrode shape, namely, a cylindrical roll body of 80 to 100 mm diameter, which therefore differs considerably from the sharp-edged electrode profile which is generally regarded as ideal.
The device proposed in German Utility Model No. 74 14 967 appears to be an attempt to solve the problems mentioned above. In this device, profiled bare electrode rolls are used in combination with an insulated counter-electrode roll. There are, however, no clearly apparent advantages as compared to a stationary electrode profile (knife, comb-shaped electrode, etc.) and, additionally, the desirable principle of the insulated electrode has been abandoned. Consequently, the patented device can only be regarded as a compromise resulting from the necessity of improving a system which is not optimal and the desire to maintain a given concept.
The electrode described in German Offenlegungsschrift No. 2,556,228, on the other hand, constitutes a practically optimal device within the category of cylinder or roll electrodes. In spite of a greatly improved utilization of capacity as compared with conventional electrodes, several electrodes must still be used in the case of high treatment speeds. This may be considered as a certain disadvantage in view of the larger technical expenditure and higher cost required at the outset.
In equipment used for treating materials of greater widths, in which the diameter of the roll electrode must be increased for constructional reasons, e.g., because of deformation of the rolls, the disadvantages of the conventional cylinder or roll electrodes are far more evident. In this instance, a considerably reduced energy density is accompanied by an increase of the mass of current-carrying parts and thus of the electrical losses due to radiation, since the entire roll body is live, although only a small portion thereof is required as the discharge surface.